Clinical laboratory tests are routinely performed on the serum or plasma of whole blood. In a routine assay, red blood cells are separated from plasma by centrifugation, or red blood cells and various plasma proteins are separated from serum by clotting prior to centrifugation.
Haemoglobin (Hb), bilirubin (Bili) and light-scattering substances like lipid particles are typical substances which will interfere with, and affect spectrophotometric and other blood analytical measurements. Such substances are referred to as interferents.
Many tests conducted on plasma or serum samples employ a series of reactions which terminate after the generation of chromophores which facilitate detection by spectrophotometric measurements at one or two wavelengths. Measurement of interfering substances prior to conducting such tests is important in providing meaningful and accurate test results. In fact if a sample is sufficiently contaminated with interferents, tests are normally not conducted as the results will not be reliable.
In analytical laboratories bar codes are increasingly being used to identify samples, and such laboratories routinely analyze a variety of biologic fluids, for example, the most common being blood and urine.
Specimen integrity directly affects the accuracy of test results. Numerous factors can compromise specimen integrity such as, having the right sample eg., blood rather than urine; in the case of a blood sample, whether it is serum or plasma; the presence of interferents in a plasma or serum sample; the volume of the sample; the sample temperature; and the location of the upper surface of a gel barrier, which is also referred to herein as the gel level, in a blood sample, where the gel is an inert material used to separate serum or plasma from clotted or packed blood cells, respectively. Finally, it is critical that the sample tested be properly matched to the results of any assessments on the sample.
Current methods used for quality assurance and specimen integrity rely principally on visual inspection of the specimen with or without comparison to a reference chart, depending upon which variable is being assessed. Visual inspection of samples is sometimes employed on a retrospective basis where there is disagreement between test results and clinical status of the patient in order to help explain such discrepancies.
A sample of plasma or serum is normally transferred from the original tube to a secondary tube. These secondary tubes may be amber coloured to protect photo sensitive constituents. Amber colouring makes visual inspection virtually impossible. On occasion, labels cover portions of the tube further restricting a full visual examination. Further, it is sometimes difficult to distinguish between urine and plasma or serum samples, even in transparent tubes.
Pre-test screening of specimens by visual inspection is semi-quantitative at best, and highly subjective and may not provide the quality assurance required.
Furthermore, visual inspection of specimens is a time consuming, rate limiting process. Consequently, state-of-the-art blood analyzers in fully and semiautomated laboratories do not employ visual inspection of specimens. However, other methods such as direct sampling are not rapid enough or cost effective. In order to obtain a measurement of the sample of the plasma or serum, specimen tubes must be uncapped, a direct sample of the specimen taken and diluted prior to measurement.